1. Field of the Invention
The present invention is related to a method for noise reduction of a FM signal, in particular to such a method comprising the step of companding the difference signal of a FM signal and transmitting the compressed difference signal additionally within the (normally transmitted) FM signal.
2 . Discussion of the Background
Companders are generally known. A compander compresses the difference signal_before the channel or storage medium and expands after the channel or storage medium. Therewith, audible noise distortions which are added to the transmitted or stored signal are reduced by such a compander. One of the best known companders for tape recording purposes is the Dolby-B-type noise reduction system. Such a syllable compander calculates the slowly varying envelope amplitude of the audio signal and compresses/expands the audio signal according thereto. A detailed description of companders and in particular of the Dolby NR (Noise Reduction) system can be found under “http://www.dolby.com/ken”.
Further, the usage of a compander for FM broadcast is also generally known. In this field a noise reduction of the difference signal noise is achieved by compressing the difference signal in the transmitter and transmitting the compressed difference signal additionally within the normally transmitted FM signal. According to Emil L. Torick and Thomas B. Keller “Improving the signal-to-noise ratio and coverage of FM stereophonic broadcasts”, J. Audio Enc. Soc., Vol. 33, No. 12, New York, December 1985, pages 938-943, presented under the title “FMX Studio Broadcast System” at the 79th convention of the Audio Engineering Society, Oct. 12-16, 1985, the compressed difference signal is added to the in-quadrature component of the modulated 38 kHz carrier, i.e. the compressed difference signal is transmitted in quadrature to the uncompressed difference signal. Alternatively, DE 41 28 045 A1 describes to add the compressed difference signal to the lower sideband of the modulated 38 kHz carrier and to subtract the compressed difference signal from the upper sideband of the modulated 38 kHz carrier before transmission of the so modified multiplex signal. Both modulation systems are backward compatible to the existing FM-Standard. A mathematical analysis of both described modulation systems leads to the result that the modulation system described in DE 41 28 045 A1 leads to less distortions in conventional FM receivers than the modulation system described in Emil L. Torick and Thomas B. Keller.
Both companders are auxiliary controlled companders which are controlled by the conventional difference signal. The conventional difference signal is defined as the difference signal decoded by a conventional FM receiver, i.e. the DSB modulated difference signal. Both compressors do not exploit the availability of the auxiliary signal for the minimization of transient overshoots in the compander. Transient overshoots might lead to an overmodulation of the FM transmitter. This overmodulation is not allowed and must therefore be avoided.